EDP Sciences
Free access
Volume 413, Number 3, January III 2004
Page(s) L23 - L26
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20031690

A&A 413, L23-L26 (2004)
DOI: 10.1051/0004-6361:20031690


Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop

B. Kliem1, V. S. Titov2 and T. Török1, 3

1  Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany
2  Theoretische Physik IV, Ruhr-Universität Bochum, 44780 Bochum, Germany
3  School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife KY16 9SS, UK

(Received 4 April 2003 / Accepted 3 November 2003 )

We study dynamical consequences of the kink instability of a twisted coronal flux rope, using the force-free coronal loop model by 1999 as the initial condition in ideal-MHD simulations. When a critical value of the twist is exceeded, the long-wavelength ( m = 1) kink mode develops. Analogous to the well-known cylindrical approximation, a helical current sheet is then formed at the interface with the surrounding medium. In contrast to the cylindrical case, upward-kinking loops form a second, vertical current sheet below the loop apex at the position of the hyperbolic flux tube (generalized X line) in the model. The current density is steepened in both sheets and eventually exceeds the current density in the loop (although the kink perturbation starts to saturate in our simulations without leading to a global eruption). The projection of the field lines that pass through the vertical current sheet shows an S shape whose sense agrees with the typical sense of transient sigmoidal (forward or reverse S-shaped) structures that brighten in soft X rays prior to coronal eruptions. The upward-kinked loop has the opposite S shape, leading to the conclusion that such sigmoids do not generally show the erupting loops themselves but indicate the formation of the vertical current sheet below them that is the central element of the standard flare model.

Key words: instabilities -- magnetic fields -- MHD -- Sun: corona -- Sun: coronal mass ejections (CMEs) -- Sun: flares

Offprint request: B. Kliem, bkliem@aip.de

© ESO 2004